The systematic development and application of mechanism maps for high-manganese steels on the basis of thermodynamic calculations was the initial goal of current investigation. By using the available datasets, variety of composition- and temperature-dependent maps for controlling the deformation mechanisms in TRIP/TWIP and TWIP systems were developed. The stacking fault energy, the change of Gibbs free energy due to TRIP mechanism with its chemical and magnetic contributions, and finally the composition-dependent equilibrium temperature for TRIP effect, were calculated and integrated in the mechanism maps. The consistency of the thermodynamically predicted mechanisms was both compared with the published data in the literature, and with the laboratory produced high-manganese steels in the Fe-Mn-C system. The observed changes in the resultant mechanical properties were explained according to the as-received microstructures, the activated plasticity mechanisms, the deformation parameters, and the interstitial and substitutional alloying contents. In addition, variations in the flow behavior of high-manganese TWIP steels within Fe-Mn-C and Fe-Mn-Al-C systems were studied by consideration of the effects of temperature, strain-rate, and grain-size-distribution. The cooperative application of three derivatives of flow curve data namely, work-hardening-rate, strain-rate-sensitivity, and instantaneous strain rate, was shown to be able to efficiently explain the occurrence of serrated and smooth plastic flow in these steels. The increase of stacking fault energy by increasing the deformation temperature and the applied strain rate, together with the strong grain-size-dependency of deformation twinning were among the most important defining parameters for the strain-hardening behavior of TWIP steels. In addition, the established roles of carbon and aluminum on changing the twins' internal structure and twins' morphology were reviewed and compared with the experimental results of the present work. Specifically, the interconnected influence of deformation twinning and dynamic strain aging on the evolution of serrated plastic flow was discussed. It was found that a proper application of thermodynamics-based mechanisms maps, together with the gathered knowledge in terms of the dependency of flow behavior on the deformation parameters, is a true strategy for tailored materials design in high-manganese steels' systems.